1. Field of the Invention
The present invention relates to a mirroring method, a mirroring device, and a computer product.
2. Description of the Related Art
Remote mirroring, which involves replicating data in a primary location at a physically remote site, is considered as a technology for providing security for the protection of critical data from disasters such as fire and earthquake.
For example, a technology is disclosed in Japanese Patent Application Laid-open No. 2003-167683 wherein, among data written at a local site, user data and a copy of journal log are transmitted to a remote site, excluding the metadata. As a result, as the metadata at the remote site is updated using the copy of the journal log and the data are updated at the local site, the update contents can be referred to in the latest file data at the remote site in real time.
In another technology disclosed in Japanese Patent Application Laid-open No. 2005-128861, a write-data sequence assigning unit that assigns the sequence of write data reception is provided as a redundant configuration to enable a destination data storage device to store the write data in the correct sequence even if the data reception time or data related to data sequence is not received for some reason.
Remote mirroring is realized by a primary data storage device that stores an original master file, and a secondary data storage device that stores a backup file. The primary data storage device and the secondary data storage device are connected by a communication channel such as a wide area network (WAN), and either device can be accessed, as they virtually function as a single storage device.
There are two types of remote mirroring, i.e., synchronous and asynchronous. In synchronous remote mirroring, the primary data storage device writes the received data to the master file in response to a write request from a data processing device and sends the data synchronously to the backup file maintained in the secondary data storage device via the communication channel, and notifying the data processing device of write completion. Specified data stored in the master file is sent to a data processing device in response to a read request from the data processing device.
On the other hand, in asynchronous remote mirroring, the primary data storage device writes the received data to the master file in response to a write request from the data processing device, but sends only file management data to the backup file in the secondary data storage device, and notifies the data processing device of write completion. The primary data storage device sends the real data to the secondary data storage device later. Specified data stored in the master file is sent to the data processing device in response to a read request from the data processing device.
The secondary data storage device receives confirmation from the primary data storage device that the file to which data is to be written in response to the write response from the data processing device is open, sends the data to the master file in the primary data storage device, synchronously receiving the replica file management data from the primary data storage device. The secondary data storage device then copies the replica file management data in its backup file and notifies the data storage device of write completion. When the real data is later received from the primary data storage device, the secondary data storage device writes it to the backup file. In asynchronous remote mirroring, if a read request is received from the data processing device before the real data is received from the primary data storage device, the secondary data storage device sends to the primary data storage device a request for the real data, writes the real data received from the primary data storage device to the backup file, and sends the specific data from the backup file.
However, each of the plurality of data storage devices deployed for realizing remote mirroring has different levels of performance when processing the request from the data processing device. In other words, every time the data processing device accesses the secondary data storage device for a write request, the secondary data storage device has to first confirm that the master file in the primary data storage device is open, making the write and read performance from the secondary data storage device a less efficient one. Thus, the data processing device experiences a disparity in the read/write speed between the primary data storage device and the secondary data storage device.